As an example of the ball joint of this type, there has been known a ball joint as disclosed in JP 09-189322 A (Patent Literature 1). The ball joint disclosed in this document includes a ball stud having a ball provided to a distal end of a shaft member so as to serve as a pivot center of a link mechanism, and a holder for covering a largest diameter portion of the ball of the ball stud so as to be held in sliding contact with a spherical surface of the ball.
The holder is molded through casting by using the ball as a core. When a bearing steel ball having high sphericity is used as the ball, a metal sliding contact surface having high sphericity, onto which the spherical surface of the ball is transferred, is formed in the holder. Further, the ball stud is formed by joining the shaft member to the ball through electric resistance welding after casting of the holder. In order to perform the electric resistance welding, welding current needs to be supplied between the ball and the shaft member. However, the holder, which is molded through casting by using the ball as a core, is held in close contact with the ball. Thus, an electrode is brought into abutment not directly against the ball, but against the holder held in close contact with the ball. In this way, the welding current is supplied between the ball and the shaft member.
Further, the holder molded as described above is held in close contact with the ball due to casting shrinkage that occurs after the casting. In this state, the ball stud cannot be pivoted with respect to the holder. As a measure therefor, in the ball joint disclosed in Patent Literature 1, an impact is applied to the ball stud in an axial direction thereof so as to expand, under pressure by the spherical surface of the ball, the metal sliding contact surface of the holder that is formed through casting. With this, a clearance is secured between the holder and the ball, and the holder and the ball are separated from each other. In this way, the ball stud can be pivoted with respect to the holder.
Meanwhile, JP 2004-316771 A (Patent Literature 2) discloses a ball joint in which a ball and a holder molded through casting by using the ball as a core are separated from each other with a resin sliding contact member. The resin sliding contact member covers a largest diameter portion of the ball of the ball stud so as to be held in sliding contact with a spherical surface of the ball. The holder covers a periphery of the resin sliding contact member, and holds the resin sliding contact member. The holder is molded through casting by using the ball covered with the resin sliding contact member as a core, and the resin sliding contact member has a resin sliding contact surface in conformity with the spherical surface of the ball and is firmly fitted to the holder. With this, the ball stud is reliably smoothly operated with respect to the holder.
Further, as in Patent Literature 1, the ball stud is formed by joining a shaft member to the ball through electric resistance welding after casting of the holder. However, the resin sliding contact member is interposed between the ball and the holder, and hence welding current cannot be supplied to the ball with use of the holder unlike Patent Literature 1. Thus, a power supply port is formed through the resin sliding contact member and the holder, and a power supply electrode is brought into direct abutment against the ball through the power supply port.
In addition, in the ball joint of Patent Literature 2, the resin sliding contact member is reheated after completion of the casting of the holder so as to reduce a squeezing force of the resin sliding contact member with respect to the ball. With this, movement of the ball stud with respect to the holder is controlled. In other words, in the ball joint of Patent Literature 2, unlike Patent Literature 1, without a need to plastically deform the holder so as to intentionally secure a clearance between the ball and the holder, smooth movement of the ball stud with respect to the holder can be achieved.